Use of a gaseous analgesic such as nitrous oxide in dental work of a potentially painful nature has become widely accepted. Nitrous oxide reduces the patient's sensitivity to pain without rendering the patient unconscious. It can be administered nasally, leaving the mouth unobstructed and free for dental treatment.
Commonly, nitrous oxide is administered using a nasal inhaler, or nosepiece, secured over the patient's nose and connected through appropriate valves to a source of nitrous oxide. A typical example can be found in U.S. Pat. No. 3,889,671 to Baker. Many devices for administering nitrous oxide gas are available on the market. Some systems provide for a constant flow of gas; others are configured with pressure demand means so that gas is supplied only when the patient inhales. A nasal cannula can also be used, delivering the flow of gas directly into the patient's nostrils.
Available nitrous oxide masks and cannulas described above, release gas to the environment. In many systems, gas exhaled through the patient's nose is directly expelled from the nosepiece to the environment through a pressure relief valve. Also, since pressurized gas inside the nosepiece tends to lift the nosepiece away from the face, gas will leak through the facial seal at the rim of the nosepiece. Finally, gas is released to the environment by exhalation through the patient's mouth. Since very little gas is absorbed and retained by the patient, nearly all the gas administered to the patient is ultimately released into the room.
Release of nitrous oxide to the environment is undesirable for two reasons. First, there is the tendency for the dentist and other dental office personnel to become anesthetized. Second, exposure to nitrous oxide is an occupational hazard; studies have shown a probable correlation between routine long term exposure to nitrous oxide and certain serious diseases. Researchers suspect that the increased rate of spontaneous abortion among female anesthetists, increased incidence of birth defects among children born to anesthetists, and higher rates of cancer and disease of the liver and kidney, is related to chronic exposure to waste nitrous oxide. Clearly, a system for administering nitrous oxide without loss of gas to the environment is desirable.
Significant reduction in nitrous oxide levels in the dental operatory may be achieved by checking equipment for leaks and by using a gas scavenging system. Several types of scavenging masks are commercially available. One variety, described in U.S. Pat. No. 4,015,598 to Brown, includes a first nosepiece disposed within a second nosepiece. Nitrous oxide is supplied to the inner nosepiece while a source of vacuum is connected to the region between the first and second nosepieces. The double nosepiece variety of mask is expensive initially and has a high replacement cost. Furthermore, the mask is difficult to keep clean and hard to disassemble; and its four hoses make it cumbersome, uncomfortable to the patient, and obstructive to the operating field of the dentist. Another type of scavenging system is the Allen circuit, made by Dupaco, San Marcos, Calif. The Allen circuit uses a gas reservoir and pressure demand valves to recirculate exhaled gases. This type of system is expensive, as it requires elaborate valving and gas controls. Another type of scanvenging mask, offered by Frazer Sweatman, Inc. Lancaster, N.Y. uses a constant flow gas supply connected to a nosepiece and a pressure operated one-way exhaust valve. When the nosepiece gas pressure rises above a predetermined level, the exhaust valve opens to scavenge gases. In a mask of the latter type, gas tends to leak out the edges because the nosepiece is pressurized. None of the previously available scavenging masks described above are effective in scavenging gas exhaled through the patient's mouth.